Electrical power tool having a motor control circuit for providing control over the torque output of the power tool

ABSTRACT

A power tool such as an electric drill typically contains a gear train that couples the output spindle of the motor to the tool bit-receiving chuck. The control circuit for the power tool is operable in a ratcheting or pulse mode that causes the output spindle to rotate in discrete incremental amounts. Corresponding methods for controlling the operation of the electric motor of a power tool are also disclosed.

This application is a continuation of application Ser. No. 08/834,774,filed Apr. 3, 1997 now U.S. Pat. No. 6,424,799, which is a continuationof application Ser. No. 08/369,497, filed Jan. 6, 1995 (now abandoned),which is a continuation-in-part of application Ser. No. 08/087,932,filed Jul. 6, 1993, now U.S. Pat. No. 5,440,215.

TECHNICAL FIELD

This invention relates to electrically driven power tools and, inparticular, to a power tool such as a variable speed drill having amotor control that is adapted to increase and better control theeffective torque output of the tool.

BACKGROUND OF THE INVENTION

Electrical power tools, such as variable speed drills and powerscrewdrivers, typically include a motor control circuit that is adaptedto control the speed of the output spindle of the tool by controllingthe amount of current supplied to the motor. The desired motor speed isusually selected by the operator by varying the position of the triggerswitch.

If the power tool is provided with an open loop motor control circuit,the speed of the output spindle of the tool will decrease as the tool isloaded and the current drawn by the motor will increase. Accordingly, ifa relatively constant output speed is desired, the operator mustmanually compensate for the reduction in motor speed as the tool isloaded by further retracting the trigger switch to increase the powerapplied to the motor. If the power tool is provided with a closed loopmotor control circuit, the control circuit is typically designed toautomatically increase the amount of power supplied to the motor as theoutput spindle of the tool is loaded in order to maintain the desiredspeed.

Thus, when employed in a power screwdriver to drive a screw into aworkpiece, for example, the current drawn by the motor will increase asthe torque required to drive the screw increases, regardless of whetherthe control circuit provides open or closed loop control. This operationwill continue until either the operator releases the trigger or themotor stalls as the increased torque required to drive the screw exceedsthe torque capacity of the tool. Consequently, the effectiveness of manyportable power tools, particularly power screw-drivers, is directlyrelated to the tool's maximum torque output capacity. Obviously, thegreater the output capacity of the tool, the more useful and versatilethe tool. However, in order to significantly increase the torque outputcapacity of a tool, it is generally regarded as being necessary(assuming changes to the gear train are not an option) to increase thesize of the motor and, consequently, the size, weight, and cost of thetool.

Accordingly, it is the primary object of the present invention toprovide a portable electrical power tool having a motor speed controlcircuit that is able to substantially increase the effective torqueoutput of a power tool for a given size motor and gear train.

In addition, it is an object of the present invention to provide aportable electric power tool, such as a power screwdriver having a motorcontrol circuit, that enables the operator to better control the torqueoutput of the tool, which is particularly beneficial when driving ascrew into a workpiece.

The motor control circuit employed in the present invention is able toachieve these objectives by intermittently pulsing the motor forpredetermined periods of time after a threshold current level isattained. More specifically, it has been found that if power to themotor of a power drill is reduced for a length of time sufficient toallow the gear train coupled to the motor to at least partially “relax”,and then power is increased, the motor is able to build up potentialenergy before the looseness (i.e., backlash) is removed in the geartrain. In effect, the motor is afforded a “running start” while the geartrain is relaxed. When the backlash in the gear train is removed, thesudden impact of the motor torque on the gear train causes a sudden andhigh burst of torque to be imparted to the output spindle of the drill,and hence to the driving bit secured thereto. When this pulse control ofthe motor is repeated, the motor is able to provide a series of burstsof torque to the gear train which in turn can be used to better finishdriving a wood screw into and below the surface of a workpiece. Thepresent control scheme thus provides better user control due to the factthat the screw does not rotate too much when static friction isovercome. Rather, with each torque pulse, static friction is overcomeand the screw is incremented a fraction of a turn.

While the pulsing operation described above has been found to beparticularly helpful and effective when used to drive wood screws andother like implements into a work surface, it has also been found to bean effective means for “breaking loose” a screw or like fastener whichis tightly seated in a workpiece, where other forms of power tools suchas conventional variable speed drills are unable to do so. By reversingthe action of the variable speed drill and applying the pulsingoperation described above, the bursts of high torque applied by themotor have been found to be extremely effective in overcoming the highlevel of stiction force required to initiate removal of such fasteners.

Accordingly, it is a further object of the present invention to providean electrically driven power tool, such as a variable speed power drill,which incorporates a control circuit for controlling a motor thereofsuch that the motor can be alternately pulsed fully on and then fullyoff at a predetermined cycle time during operation of the drill.

It is another object of the present invention to provide an electricallydriven power tool having such a control circuit that further provides anoperator of the power tool with a means for adjusting the point at whichthe alternating full-on and full-off operation is initiated.

It is yet another object of the present invention to provide anelectrically driven power tool which automatically enters thealternating full-on and full-off mode of operation when the currentthrough the motor exceeds an operator adjustable threshold levelsetting.

Additionally, it is an alternative object of the present invention toprovide an electrically driven power tool that provides the operatorwith control over the magnitude of the torque bursts during thealternating phase of operation of the tool.

Finally, it is an object of the present invention to provide simplifiedversions of the present motor control circuit that are suitable for usein relatively low-cost power tools.

SUMMARY OF THE INVENTION

The above and other objects are provided by a portable electric powertool having an electronic control circuit and method in accordance withpreferred embodiments of the present invention. The control circuit ispreferably disposed within the housing or body of the electricallydriven power tool, which is represented illustratively herein as avariable speed power drill. The control circuit generally comprisesoperator adjustable means for setting a threshold current level whichdefines a transition point at which alternating on and off operation ofthe motor is initiated; a trigger switch for selecting the desired speedof the motor; a current sensing circuit for sensing the current flowingthrough the motor; a switching circuit for controlling the flow ofcurrent to the motor; and a controller for comparing the current sensedby the current sensing circuit relative to the threshold current levelselected by the operator and controlling the switching circuit tocontrol the amount of current applied to the motor. When the currentdrawn by the motor exceeds the selected current threshold level, thecontroller is adapted to temporarily interrupt current flow for apredetermined “off-time” interval, and then reapply a maximum currentsignal for a predetermined on-time interval, and to alternate this onand off operation until the trigger switch is released.

The off-time interval during which the controller causes the switchingcircuit to temporarily interrupt current flow to the motor is sufficientto allow the gear train coupled to the motor of the power tool tosufficiently “relax” before maximum current is reapplied to the motor. Avalue representing this time duration is preferably stored in a memoryof the controller and is unique to the gear train of the particularpower tool being controlled.

By alternately applying a maximum current signal for a desired time andthen interrupting current flow for a predetermined time, the motor ofthe power tool is caused to generate successive “bursts” of torque tothe gear train of the power tool which significantly increases theeffective torque output of the power tool. This technique further hasbeen found to be extremely effective in “breaking loose” tightly setwood screws and the like, which other conventionally controlled powertools having comparable-sized motors are unable to achieve.

In several preferred embodiments of the invention, a memory is includedfor storing a plurality of predetermined “on-times” which the controlleraccesses depending on the setting of the current threshold level settingmeans. Thus, on-times of varying duration can be selected by thecontroller to precisely meet the anticipated conditions of a specificapplication.

In an alternative preferred embodiment of the present invention, thecurrent comparison performed by the controller is modified in accordancewith the changing (i.e., increasing) speed of the motor as the triggerswitch is squeezed during operation of the power tool to increase motorspeed. In this instance the threshold current level signal selected bythe operator is decreased as the speed of the motor increases. With thisembodiment a speed sensor is employed to monitor the speed of the motorand provide a signal representative thereof to the controller. As thespeed of the motor increases due to progressive engagement of a triggerof the power tool, the controller decrements the operator-selectedthreshold current level signal. This alternative embodiment furtherhelps to compensate for the inertia of the gear train at higher motorspeeds and helps provide even more consistent results independent of themotor speed of the power tool.

In yet another alternative preferred method of operation of the presentinvention, the transition point for beginning the alternating on and offoperation (referred to also as the “ratcheting model” of operation) ofthe motor is determined in accordance with a predetermined percentageincrease in the sensed motor current. With this method the currentthrough the motor is initially measured. After a predetermined timedelay, a second current measurement is made. This operation is repeatedcontinuously until the second current measurement exceeds the initialcurrent measurement by a predetermined factor. At that point thecontroller initiates the alternating on and off operation of the motor.In this embodiment the operator-adjustable threshold current level meansis replaced by a means for allowing the operator to adjust the desiredon-time of the motor once the ratcheting mode of operation has begun.

This embodiment and method of operation thus provides a method for“automatically” sensing the size of a screw (and thus the torquerequired to drive the screw) as the operator begins driving the screwinto a workpiece, based on the initial current reading. Since thecurrent required to drive a large screw is greater, in the initialstage, than that required for a small screw, setting the transitionpoint in accordance with a predetermined increase in current (e.g., 25%or 50%) automatically serves to adjust the transition point at which theratcheting mode of operation begins in accordance with the size of thescrew being driven.

In yet another alternative preferred mode of operation of the presentinvention, the transition point is determined by a predetermined drop inmotor speed. In this embodiment, the ratcheting mode of operation of themotor is initiated when the motor speed drops below a predeterminedspeed, or by a predetermined amount (i.e., percentage), or by apredetermined rate.

In a further alternative embodiment, it has been determined to beadvantageous to provide the operator with control over the magnitude ofthe torque bursts during the ratcheting mode of operation. In otherwords, rather than providing fixed on-time/off-time periods during theratcheting mode of operation, it may also be desirable to provide theoperator with the ability to continue to vary the duty cycle of thevoltage signal during the ratcheting mode of operation in accordancewith the position of the trigger switch.

In particular, conventional variable speed power tools control the speedof the motor by varying the duty cycle of the voltage signal supplied tothe motor. The frequency of the duty cycle signal is set sufficientlyhigh—typically 1 KHz to 12 KHz—so that the motor operates smoothly eventhough the power is actually being rapidly cycled on and off. Thepercentage on-time of the duty cycle signal, and hence the average powerlevel, supplied to the motor is controlled by the operator in accordancewith the position of the trigger switch.

Consequently, it will be appreciated that the transition from normalvariable speed control of the motor to the above-described ratchetingmode of operation can be viewed simply as a change in the frequency ofthe duty cycle control signal. In other words, the ratcheting mode ofoperation can be achieved simply by switching from a relatively highfrequency control signal to a relatively low frequency control signal(e.g., 10-50 Hz), the period of which is greater than the response timeof the motor. Considered in this manner, it is readily apparent that itis possible to continue to provide trigger switch control over the dutycycle of the control signal during the ratcheting or low frequency modeof operation, and thereby provide the operator with the ability tocontrol the magnitude of the torque bursts. This, in turn, provides theoperator with greater control when seating a screw into a workpiece.

Lastly, various simplified versions of the present invention aredisclosed. In these alternative embodiments, the motor control circuitdoes not automatically transition between conventional variable speedcontrol and the low frequency pulse mode of operation. Consequently, themore sophisticated microcomputer-based controller, as well as thefeedback circuitry for sensing motor current and speed, can beeliminated.

In a first version of this simplified form of the present invention, thepower tool is continuously operated in the low frequency pulsing mode.In particular, it has been found that even at a low duty cyclefrequency, such as 10 to 50 Hz, the speed of the motor can be varied byvarying the duty cycle of the control signal to the motor. Moreover,because the off periods are relatively short (typically less than 10msec.) at high duty cycle settings, the application of torque from themotor to the output spindle of the tool is relatively smooth. However,as the duty cycle signal is reduced, the off periods between successivepulses increase, thereby producing a more pronounced pulsing operation.This form of motor speed control is thus particularly advantageous whencontrolling a drill to drive screws. In particular, an operatortypically operates a conventional variable speed drill at or near fullpower (100% duty cycle) during the initial stage of driving a screw andthen slows the motor (low duty cycle) during the final stage when thehead of the screw is being seated to the proper depth. This technique istherefore readily compatible with the described simplified form of motorspeed control as the motor operates in a conventional manner at fullpower with the pulsing action becoming more apparent at lower duty cyclesettings. Thus, the operator is provided with significantly improvedcontrol over the depth to which the screw is set. Moreover, despite therelatively low duty cycle settings, the pulsing action produces enhancedbursts of torque which drive the screw in controllable incrementalamounts, thereby permitting the operator to accurately set the depth ofthe screw.

In an additional alternative embodiment of the simplified version of thepresent invention, a selector switch is provided for enabling theoperator to switch between a normal high frequency “drilling” mode ofoperation and a low frequency “screw driving” mode of operation.

Finally, a further alternative embodiment of the simplified version ofthe present invention is disclosed that provides a separately adjustablecontrol knob for varying the frequency of the PWM signal. Moreparticularly, the trigger switch in this embodiment functions in aconventional manner to control the duty cycle of the PWM signal. Anadditional operator actuable knob or dial, preferably located on the topof the drill, is provided for selectively setting the frequency of thePWM signal. Thus, for example, when driving small screws requiring morecontrol, a moderately low frequency (e.g., 50 Hz) can be used whichreduces the maximum off time between successive pulses and therebylimits the magnitude of the torque spikes applied to the screw. However,when setting large screws, a lower frequency (e.g., 10 Hz) can beselected which results in sufficiently long off periods to enable thegear train to completely relax, thereby allowing the subsequent build-upof greater potential energy and the application large torque bursts tothe screw. Additionally, the operator can simply set the frequency to anormal high PWM frequency level (e.g., 12 KHz) for operation as aconventional variable speed drill.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone skilled in the art by reading the following specification andsubjoined claims and by referencing the following drawings in which:

FIG. 1 is an elevational side view of a typical variable speed powerdrill with which the control circuit of the present invention may beused;

FIG. 2 is a simplified block diagram of a preferred embodiment of thecontrol circuit of the present invention;

FIG. 3 is a flowchart of the steps of operation performed by the controlcircuit in implementing the alternating on and off, or “ratcheting” orlow frequency, mode of operation;

FIG. 4 is a graph of the current flow through the motor verses the depthof a screw being driven in by a power tool incorporating the controlcircuit of the present invention while a trigger of the power tool isheld steady in an engaged position;

FIG. 5 is a flowchart of an alternative method of control fordetermining the transition point as to when the ratcheting mode ofoperation is to begin;

FIG. 6 is a flowchart of another alternative method of control in whichthe current through the motor of the power tool is measured repeatedlyand the ratcheting mode is implemented when the current increases by apredetermined factor from an initial measurement;

FIG. 7 is a flowchart of another alternative preferred method of controlfor determining an appropriate transition point by sensing for apredetermined drop in motor speed;

FIG. 8 is a partial flowchart diagram of an alternative manner ofcontrolling the power tool during the ratcheting or low frequency modeof operation illustrated in FIG. 3;

FIG. 9 is a circuit diagram of a simplified version of a motor controlcircuit incorporating the teachings of the present invention;

FIG. 10 is a diagram graphically illustrating the relationship betweenoutput torque and motor rotation for a typical power drill;

FIGS. 11a-11 c are timing diagrams illustrating the operation of thepresent motor control circuit at various low frequencies;

FIG. 12 is an additional alternative embodiment of the simplifiedversion of the present motor control circuit shown in FIG. 9; and

FIGS. 13a and 13 b are timing diagrams illustrating the operation of themotor control circuit shown in FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an electrically driven power tool in the form of acordless (i.e., battery driven) variable speed power drill 10incorporating an electronic control circuit 12 in accordance withpreferred embodiments of the present invention is illustrated. As willreadily be appreciated by those skilled in the art, the motor controlcircuit taught by the present invention is adaptable to other types ofelectrical power tools, such as power screwdrivers, electric pop rivetguns, and the like. The control circuit 12 includes a current thresholdpotentiometer 14 which is disposed in a convenient position on the drill10 to allow an operator to conveniently adjust the potentiometer 14 asneeded. The function of this potentiometer 14 will subsequently bedescribed in greater detail.

Optionally included is an on-time adjustment potentiometer 15 which maybe used in lieu of potentiometer 14 in an alternative preferredembodiment of the present invention. The on-time potentiometer 15provides an operator with direct control over the on-time intervalimplemented during the “ratcheting” mode of operation of the motor 16.It will be appreciated, however, that potentiometers 14 and 15 couldeasily be used simultaneously to provide the operator with control overthe point at which the ratcheting mode of operation begins as well asthe duration of the on-time, if such is desired.

The drill 10 includes, in conventional fashion, a motor 16 and arechargeable battery 18 for powering the motor 16. While the drill 10has been illustrated as battery powered, it will be appreciated that thecontrol circuit 12 of the present invention could just as easily be usedwith an A/C powered drill with little or no modification, providedsuitable phase control circuitry is included.

The motor 16 of the drill is adapted to drive through a conventionalgear train 20, a tool bit-receiving chuck 22. A trigger switch 24controls the battery voltage across the motor 16 and therefore thecurrent flowing through the motor 16 to provide an operator with thecapability of varying the speed of the chuck 22 to suit various workneeds.

With reference to FIG. 2, a block diagram of a preferred embodiment ofthe control circuit 12 is illustrated. The control circuit 12 is used toimplement the alternating on and off operation of the motor 16, whichwill hereafter be referred to as the “ratcheting” mode or low frequencymode of operation.

The control circuit 12 generally includes a controller 26 in the form ofa microcomputer, the threshold current level potentiometer 14, a memorydevice 28 such as a read-only memory (ROM), and a current sensingcircuit 30 coupled in series with the motor 16. A switching circuit inthe form of a metal oxide silicone field effect transistor (MOSFET)drive circuit 32 is provided for controlling the voltage applied acrossthe motor 16 and thus the current flow through the motor 16, inaccordance with the duty cycle of the pulse width modulated (“PWM”)control signal received from the microcomputer 26. The on-timeadjustment potentiometer 15 is also shown in phantom indicating itspresence as being optional.

The controller circuit 12 further includes the trigger switch 24 whichprovides a signal to the microcomputer 26 in accordance with the degreeof retraction thereof by the operator. A DC battery pack 18 and aconventional voltage regulator 34 connected across the battery areprovided for supplying a regulated DC voltage across the motor 16 and tothe microcomputer 26.

The control circuit 12 may optionally include a speed sensor 36 forsensing the speed of the motor 16 and providing a signal in accordancetherewith to the microcomputer 26. The speed sensor 34 may take the formof a variety of well-known speed sensing devices such as opticalencoders or Hall-effect sensors which are capable of supplying a seriesof pulses to the microcomputer 26 which are representative of thefrequency of rotation of the motor 16. The use of the speed sensor 34will be described in greater detail hereinafter in connection with analternative preferred embodiment of the present invention.

The microcomputer 26 preferably is comprised of an 8-bit microprocessorwhich includes an on-board memory 29, preferably in the form ofread-only memory (ROM), for storing, inter alia, a constant which in thepreferred embodiment represents an “off-time” duration sufficient toallow the gear train 20 of the drill 10 to completely relax aftercurrent flow to the motor 16 is interrupted. The microcomputer 26 alsoaccesses the memory 28 to read a plurality of values stored in a look-uptable therein which represents varying on-time intervals for leaving themotor 16 fully on when the ratcheting mode is implemented. Finally, themicrocomputer 26 is responsive to the threshold current levelpotentiometer 14 to provide an operator with the capability of adjustingthe point at which the ratcheting mode of operation is to begin. Thispoint will be referred to hereafter as the “transition” point.

It should be appreciated that the relax time, and thus the desiredoff-time, will vary from tool to tool depending on the design of thegear train. Accordingly, power tools such as drills having gear trainsof differing design will most likely require different off-times toallow their gear trains to completely relax. The off-time for a specificgear train design may be determined by any suitable testing procedurewhich provides a relatively accurate determination of the time intervalrequired, once torque is removed from the gear train, for the gear trainto return to a “relaxed” condition. With the preferred embodimentsdescribed herein, an off-time within a range of about 20 ms to 100 msprovides sufficient time for the gear train to completely relax,although this can vary greatly depending upon the particular type oftool.

In addition, it will be appreciated that it is not critical to thepresent invention that the duration of the off-time period be sufficientto enable the gear train of the power tool to completely relax. Inparticular, if the off-time period is such that the gear train isallowed to only partially relax, an enhanced torque effect willnonetheless be realized when power is reapplied, but simply to a lesserdegree than if the gear train is allowed to completely relax.

With particular reference to FIG. 10, a graph illustrating therelationship between motor rotation and torque for a typical power toolis shown. The gear train in a commercial power tool, such as a drill,has an inherent “looseness” due to normal manufacturing tolerances,which may allow the tool spindle to be freely rocked back and forth overa given range, typically between 30°-90°. However, due to the gearreduction in the gear train, with the output spindle locked againstrotation, the motor armature may be rotatable over a much greater range,typically in excess of two complete revolutions. This is demonstrated bythe flat portion 126 of the curve centered around the vertical axiswhich represents 2.2 complete revolutions of the motor armature.Consequently, when the motor of the tool is initially started there isactually a lag between the point in time when the motor armature beginsto rotate and when the tool spindle begins to rotate. In addition, asthe tool spindle is loaded and the torque output of the motor increases,the gear train will “tighten” further. This is demonstrated by therising portions 128 a and 128 b of the curve.

However, when power to the motor is reduced or interrupted and thetorque applied by the motor output shaft drops substantially below theloading on the spindle, the gear train will begin to untighten or“relax”. In fact, if the output torque of the motor is permitted to dropsubstantially to zero, the relaxation of the gear train will actuallydrive the motor armature in the reverse rotational direction. In otherwords, the relaxation of the gear train as motor torque collapses willdrive the motor in the reverse rotational direction down curve portion128 a, back through zero along flat portion 126, and possibly evenslightly down curve portion 128 b.

Consequently, it can be appreciated that the greatest enhanced torqueeffect can be realized by interrupting power to the motor for a periodof time sufficient to substantially coincide with the point of maximumreverse rotation of the motor armature. This optimum off-time period canreadily be determined empirically for any given tool by connecting atorque transducer to the output spindle of the tool and selectivelyvarying the off time between pulses until the greatest torque peak isachieved. This is due to the fact that further increases in the off-timeperiod beyond the optimum period only serves to unnecessarily delay thereapplication of power and does not further increase the magnitude ofthe subsequent torque pulse.

However, it will additionally be recognized that there is no singleoff-time period that is optimum for all conditions under which aparticular tool may be used. For example, when using a drill to driverelatively small screws, it may not be desirable to generate largetorque pulses that make it difficult to accurately set the screw.Rather, relatively shorter off-time periods and the re-application ofless than full power may be preferred in such applications to providecontrolled, incremental rotation of the screw enabling accurate finalsetting of the screw head relative to the surface of the workpiece. Inother words, in applications where the maximum torque required tocomplete the task is less than the torque capacity of the tool (i.e.,referred to as the “locked rotor torque”), it may nonetheless beadvantageous to operate the tool in the “ratchet mode” for bettercontrol. In short, the present invention can be advantageously employednot only in applications requiring enhanced torque output, but also inapplications requiring improved torque control. Consequently, whenemploying the control techniques of the present invention toapplications primarily for the purpose of improved torque control, suchas the setting of small screws, it is not necessary that the gear traincompletely relax between pulses or even that the motor torque actuallydrop to zero between pulses. Rather, the improved control advantages ofthe present invention can be obtained by controlling the motor in amanner that causes uneven, incremental rotation of the tool outputspindle.

Turning now to FIGS. 11a-11 c, a series of exemplary timing diagrams isshown to illustrate the control techniques taught by the presentinvention as applied to the task of setting different sized screws. FIG.11a is representative of a suitable control technique for setting arelatively small screw requiring less than the maximum torque capacityof the tool. Note that due to the limited torque requirements of thisapplication, it is not necessary that the torque output of the motoractually drop to zero between pulses. Rather, the reduction in motortorque below the loading on the tool spindle (i.e., the dynamic frictionlevel) is sufficient to substantially reduce and even temporarily haltthe rotation of the output spindle of the tool. The subsequent momentaryincrease in power output above the static friction level thus serves toincrementally rotate the output spindle. This repetitive pulsing of themotor continues until the operator releases the trigger. Consequently,the operator is readily able to accurately control the final setting ofthe screw.

FIG. 11b is representative of a control scheme for setting a largerscrew requiring the application of torque nearer to the capacity of thetool. In this application, power to the motor is sufficiently reducedbetween pulses for the torque output of the motor to drop closer tozero. Consequently, a degree of gear train relaxation is achieved,enabling the subsequent generation of enhanced torque pulses toincrementally drive the screw until properly set.

Lastly, FIG. 11c is representative of a control scheme suitable forsetting a very large screw that is otherwise beyond the capacity of theparticular tool. In this instance, the pulsing mode utilizes off-timeperiods of sufficient duration to enable the gear train to completelyrelax to maximize the magnitude of the subsequent torque pulses andsuccessfully complete the setting of the screw.

With reference to FIG. 3, a description of the operation of a preferredembodiment of the control circuit 12 will now be provided. Initially,the microcomputer 26 reads the current threshold level selected by theoperator via the current threshold potentiometer 14, as indicated atstep 38, and waits for the operator to activate the trigger switch, asindicated at step 40.

If the test at step 40 proves true, the microcomputer 26 controls theMOSFET drive circuit 32 to provide a voltage signal across the motor 16which is proportional to the degree of engagement of the trigger 24, asindicated at step 42. The microcomputer 26 then reads the output fromthe current sensing circuit 30 to determine if the motor current isgreater than the current threshold signal provided by the currentthreshold potentiometer 14, as indicated at step 44. If this test provesfalse, then steps 42 and 44 are repeated until step 44 proves true.

When the test at step 44 proves true, the microcomputer 26 accesses thelook-up table stored in the memory 28, as indicated at step 46, toobtain the appropriate on-time to be used during the ratcheting mode ofoperation. The microcomputer 26 then causes the MOSFET drive circuit 32to interrupt current flow to the motor 16 for the predeterminedoff-time, as indicated at step 48. When the off-time interval hasexpired, the microcomputer 26 causes the MOSFET drive circuit 32 toreapply a maximum current flow to the motor 16 for the predeterminedon-time interval, as indicated at step 50. As discussed previouslyherein, the off-time interval is preferably of a duration sufficient toallow the gear train 20 to completely relax.

When the on-time interval has expired, the microcomputer 26 again checksto determine if the trigger 24 is still engaged, as indicated at step52. If this test proves true, then steps 48-52 are repeated until thetest at step 52 proves false. When the test at step 52 proves false,indicating that the work operation is complete, the power to the motoris disconnected and the program returns to the start.

In certain of the preferred embodiments of the present invention, themaximum current signal is a current signal which is sufficiently largeto drive the motor at or near its maximum rated speed. This currentsignal is further applied and removed in a rapid, pulse-like fashionsuch that the motor 16 “sees” virtually instantaneous “turn-on” and“turn-off” signals.

Optionally, a plurality of varying on-times may be stored in the memory28 to enable the length of time during which the maximum current signalis applied to be correlated more precisely to the setting of the currentthreshold potentiometer 14. For example, if the transition point is setto occur at about 80% of maximum rated current flow, then a shorteron-time may be desirable than that required if the transition point isset to 90% of maximum rated current flow. Thus, by varying the on-timeinterval in accordance with the current threshold potentiometer 14, theduration of the on-time can be chosen by the microcomputer to maximizethe torque producing capability of the motor 16 to suit the needs ofspecific applications.

Referring to FIG. 4, a graph shows an exemplary current flow through themotor 16 (and thus the torque generated by the motor 16) as regulated bythe control circuit 12 when installing a wood screw completely into apiece of wood. Initially, the current flow through the motor 16 issubstantially continuous, as represented by curve 54. In actuality, dueto the relatively high frequency of the PWM control signal, there existsa corresponding high frequency ripple in the motor current during thismode of operation. When the motor current exceeds the threshold currentlevel 56 set via the current threshold potentiometer 14, whichrepresents the transition point 58, the ratcheting or low frequency modeis initiated. Current flow to the motor 16 is quickly interrupted forthe predetermined off-time 60 to allow the gear train 20 in thepreferred embodiment to completely relax. After this time interval hasexpired, the microcomputer 26 causes the MOSFET drive circuit 32 torapidly apply maximum current flow to the motor 16. This maximum currentflow is maintained for the on-time 62 read from the look-up table in thememory 28. The cycle is then repeated until such time as themicrocomputer 26 detects that the trigger switch 24 of the drill 10 hasbeen released, as indicated by portion 64 of the waveform.

Importantly, it will be appreciated, for purposes of the scope of thepresent invention as described herein and as claimed, that it isirrelevant whether the ratcheting mode of operation is initiated by an“off” time period, as shown in FIG. 4, or an “on” time period.Consequently, when the control circuit is described herein as initiallyinterrupting power to the motor in response to the detection of thetransition point, it is to be understood that the control circuit couldjust as readily initially apply full power to the motor in response tothe detection of the transition point.

Alternatively, it is readily possible to modify the software algorithmillustrated in FIG. 3 so that the operator of the power tool is able tocontrol the magnitude of the torque bursts during the low frequency orratcheting mode of operation. In particular, rather than providing fixedon-time and off-time G periods as shown in FIG. 3, the microcomputer 26can be programmed to merely reduce to a relatively low level thefrequency of the PWM control signal and continue to set the motorvoltage proportional to the position of the trigger switch. Thisalternative control scheme is illustrated in FIG. 8. In this embodiment,during the low frequency mode, the percentage on-time of the duty cyclesignal, and hence the average motor voltage signal, supplied to themotor is set in accordance with the position of the trigger switch 24.Thus, following detection of the transition point, the microcomputer 26reduces the frequency of the PWM control signal at step 45 to apredetermined relatively low level, typically between 10-50 Hz. Thisfrequency level is selected to be sufficiently low such that the periodof the PWM control signal is substantially greater than the responsetime of the motor 16. In particular, the period of each cycle of the PWMcontrol signal during the low frequency mode of operation is preferablysufficiently greater than the response time of the motor to enable theoutput spindle of the power tool to substantially slow down or even stoprotating during the off-time portion of the cycle, which, of course,will be something less than the total time period of each cycle,depending on the position of the trigger switch. In other words, at step47 the microcomputer is programmed in this embodiment to set thepercentage duty cycle of the PWM control signal in accordance with theposition of the trigger switch. This will produce corresponding on-timeand off-time periods which, added together, equal the period of onecycle of the PWM control signal. Consequently, the duration of theoff-time portion of each cycle should be long enough, at least atmoderate trigger switch settings, to cause the output torque of themotor to drop below the level necessary to rotationally drive the toolspindle, given the present loading on the tool spindle. In a typicalvariable speed drill, a frequency of 10-50 Hz has been found to beacceptable, although this too can vary depending upon thecharacteristics of a particular tool and the application involved.

In this embodiment, therefore, the operator is able to control themagnitude of the torque bursts, and thus control the rate at which ascrew is seated into a workpiece, by varying the position of the triggerswitch. Accordingly, the operator can, for example, achieve a quarterturn or a half turn of the screw with each torque burst depending uponthe position of the trigger switch, and thus properly seat a screw intoa workpiece in a very controlled manner. Consequently, the presentinvention avoids the dilemma of risking the over-application of a largeburst of power to finish setting a screw and inadvertently causing thescrew to be set too deeply below the surface of the workpiece.

Referring now to FIGS. 5-7, various alternative methods for determiningthe appropriate time to transition from the high frequency mode to theratcheting or low frequency mode of operation are disclosed. Withinitial reference to FIG. 5, the control circuit 12 in this embodimentnot only monitors the current flowing through the motor 16 to detectwhen the transition point has occurred, but also incorporates the use ofthe speed sensor 36 (shown in FIG. 2) to modify the threshold currentlevel signal provided by the current threshold potentiometer 14 as setby the operator. In particular, the suitability of a particular currentthreshold is dependent upon the speed of the motor when the threshold isattained. In other words, as motor speed is increased, the amount ofinertia in the system increases which will cause a screw to continue toturn after the motor has been turned off. Consequently, in order toprovide consistent results, it is preferable to adjust the currentthreshold in accordance with the speed of the motor at a particularpoint during the screw setting process that is relatable to theprojected speed of the motor when the threshold is attained.

As set forth in the flowchart diagram, the microcomputer 26 initiallyreads the current threshold potentiometer 14, as indicated at step 66,and waits for the operator to actuate the trigger switch 24, asindicated at step 68. Once the microcomputer 26 detects that the trigger24 has been pulled, the appropriate motor voltage is set proportional tothe trigger setting, as indicated at step 70.

The microcomputer then waits a predetermined time period, as indicatedat decision step 72. Once this time period has elapsed, the speed of themotor (V) is read at step 76, and then the current threshold level isadjusted based upon the actual speed of the motor (V) at this point andthe setting of the current threshold potentiometer 14, as indicated atstep 78. Next, a flag is set (step 79) so that the adjustment process isnot repeated (decision step 74) and the program continues in the loopuntil the motor current exceeds the adjusted current threshold level 80.

If the test at step 80 proves true, then the microcomputer 26 accessesthe look-up table in the memory 28 to determine the appropriate on-time,as indicated at step 82, to be applied during the ratcheting mode ofoperation. The microcomputer 26 then begins the ratcheting mode byturning the motor 16 full-off for the off-time, as indicated at step 84,and then turning the motor 16 full-on for the selected on-time, asindicated at step 86. Another check is then made to determine if thetrigger 24 is still being pulled by the operator, as indicated at step88. If this test proves true, then steps 84, 86, and 88 are repeateduntil the test at step 88 proves false, whereupon power to the motor isterminated.

The alternative method of control set forth in FIG. 5 thus provides ameans by which the transition point can be modified proportionally withdifferences in motor speed. This allows the control circuit 12 tocompensate for the inertia generated at high motor speeds whichcontinues to apply torque to the screw after the motor 16 is turned off.Accordingly, this method can provide even more consistent results indetermining the most effective transition point independent of how fastthe motor 16 is being operated.

Referring now to FIG. 6, another alternative method of control fordetermining the appropriate transition point is set forth. This methodessentially involves monitoring the current flow through the motor 16 todetermine when the current flow has increased by a predetermined factor(for example, doubled or tripled), for signalling the microcomputer 26to implement the ratcheting mode of operation. With this method ofcontrol the optional on-time potentiometer 15 (FIGS. 1 and 2) may beincorporated to provide direct operator control over the on-timeinterval during the ratcheting mode of operation in lieu of the currentthreshold potentiometer 14.

Once the operator has actuated the trigger switch (step 92), the on-timepotentiometer 15 is read, as indicated at step 90, and the motor voltageis set proportional to the setting of the trigger 24, as indicated atstep 94. After a first predetermined time interval (T1), a first currentflow reading I₁ through the motor 16 is taken, as indicated at step 96.The microcomputer 26 then waits a second predetermined time interval(T2), as indicated at step 98, before taking a second current reading I₂through the motor 16, as indicated at step 100. A test is then made bythe microcomputer 26 to determine if I₂ is greater than I₁ by apredetermined factor, as indicated at step 102. If this test provesfalse, the microcomputer 26 checks to see if the trigger 24 is stillpulled, as indicated at step 104 and, if so, repeats steps 98, 100, and102 until the test at step 102 proves true.

When the test at step 102 proves true, the microcomputer 26 accesses thememory 28 to obtain the appropriate on-time value from the look-uptable, as indicated at step 106. The microcomputer 26 then controls theMOSFET drive circuit 32 to interrupt current flow to the motor 16, asindicated at step 108, thus initiating the ratcheting mode of operation.

The current flow is interrupted for the preset off-time duration, afterwhich a maximum current flow signal is applied to the motor 16 for theselected on-time, as indicated at step 110. The ratcheting mode ofoperation is repeated until the microcomputer 26 detects that thetrigger switch 24 has been released, as indicated at step 112, whereuponthe power to the motor is terminated.

The alternative method of control described above in connection withFIG. 6 provides a method for determining the transition point which also“automatically” senses the size of the screw being installed based onthe first current reading at step 96. Accordingly, this method has theadvantage of automatically tailoring the transition point to occur at anappropriate time to accommodate different size wood screws. As will bereadily appreciated by those skilled in the art, the program illustratedin FIG. 6 can also be readily modified to detect a predeterminedpercentage increase in motor current or a predetermined rate of increasein motor current as the transition event before switching to the ratchetmode of operation.

Yet another alternative method of determining the transition point isshown in connection with FIG. 7. The steps shown in FIG. 7 may beimplemented in lieu of steps 96-102 of the method described inconnection with FIG. 6. With reference to FIG. 7, after steps 90-95 ofFIG. 6 have been performed, the motor 16 speed V₁ is read as indicatedat step 114. The microcomputer 26 then waits a predetermined timeinterval (T2), as indicated at step 116, before again reading the motor16 speed V₂, as indicated at step 118. A test is then made to determineif the motor 16 speed has decreased a predetermined amount (for example,by 50 percent), as indicated at step 120. If this test proves false, acheck is made to determine if the trigger 24 is still being pulled, asindicated at step 122. If this test proves false, the method loops backto the very start of the program as indicated in FIG. 6.

If the test at step 122 proves true, then steps 116 through 120 arerepeated. Once the test at step 120 proves true (i.e., the motor 16speed has decreased by a predetermined amount) the ratcheting mode ofoperation is implemented in accordance with steps 106-110 of FIG. 6.Optionally, of course, the test at step 120 could be modified to detecta predetermined percentage drop in motor speed or a predetermined rateof deceleration.

By the method described in connection with FIG. 7, a relatively simplesequence of operation is provided for determining an appropriatetransition point at which the ratcheting mode of operation is to occurwhich also takes into account the size of the wood screw being driven,as well as the hardness of the wood into which the wood screw is beingdriven. By sensing for a predetermined amount or percentage drop inmotor speed, the ratcheting mode can be implemented at appropriate timesfor a variety of applications to optimize the effectiveness of the tool.

The method of FIG. 7 also does not require the use of the currentthreshold potentiometer 14. Moreover, the on-time potentiometer 15 isalso not required for this preferred method of control.

Turning now to FIG. 9, a simplified version of a motor control circuitfor a power tool according to the present invention is shown. Inparticular, it has been determined that many of the advantages of thepresent invention can be achieved with a simplified version of the motorcontrol circuit that does not automatically transition from aconventional high frequency variable speed control mode to a lowfrequency pulse control mode. Consequently, the feedback circuitry forsensing motor current and speed can be eliminated, and a conventional555 timer-based controller can be substituted for the more sophisticatedand expensive microcomputer-based controller shown in FIG. 2.

In particular, the controller shown in FIG. 9 includes a conventional555 CMOS timer 130 that is controlled by the charging and dischargingcycles of the external RC timing circuit comprised of trigger resisterR1 and selectively connected capacitors C1 and/or C2. A user-operatedselector switch 136, positioned in a readily accessible location on thetool, is provided to selectively connect the capacitor C2 into or out ofthe control circuit. The 555 timer circuit 130 is adapted to produce aHI signal at output pin 3 when the signal applied to its TRIGGER input(pin 2) is less than 33% of the supply voltage (V+). A LO output signalis produced at output pin 3 when the signal supplied to the THRESHOLDinput (pin 6) is greater than 66% of the supply voltage (V+). When theOUT terminal (pin 3) is HI, the MOSFET 132 is rendered conductive andthe motor 16 is energized via the reversing switch 134.

In operation (and assuming for the moment that selector switch 136 is inthe open position or “DRILL” mode), capacitor C1 is initially dischargedand therefore the output signal at TRIGGER input (pin 2) is less than33% of battery voltage (+V). Accordingly, output pin 3 is HI, turning onMOSFET 132 and energizing the motor 16. With output (pin 3) HI,capacitor C1 is charged through diode D1 and trigger resistor R1. Therate at which capacitor C1 charges is, of course, determined by thevalue of capacitor C1 and the setting of trigger switch R1. Whencapacitor C attains a charge that exceeds 66% of battery voltage (+V),the output (pin 3) will go LO, thereby turning off MOSFET 132 andde-energizing the motor 16. At this point, capacitor C1 will ceasecharging and begin to discharge through trigger resistor R1, diode D2,and into the OUT terminal (pin 3) of the 555 timer 130 which is now atground potential. The rate at which capacitor C1 discharges is similarlydetermined by the value of capacitor C1 and the setting of triggerresistor R1. When the charge on capacitor C1 drops below 33% of batteryvoltage (+V), the OUT terminal (pin 3) will again go HI repeating thecycle.

Thus, it will be appreciated that the control circuit produces asubstantially square wave output signal at OUTPUT pin 3, the duty cycleof which is controlled by the setting of trigger resistor R1. Note,however, that the frequency of the duty cycle signal remains constant asa change in the position of trigger resistor R1 increasing the chargetime of the capacitor C1 causes a corresponding decrease in thedischarge time of the capacitor C1, and vice versa. In other words, ifthe duration of the HI signal at pin 3 is increased by a given amount,the duration of the corresponding LO signal is decreased by the sameamount. Consequently, the frequency of the output signal, which isdetermined by the combined period of the HI and LO signals, remains thesame.

In the preferred embodiment, the value of capacitor C1 is selected toprovide a relatively high duty cycle frequency in the range of 1-12 KHz.Accordingly, when selector switch 136 is in the open position or “DRILL”mode shown, the power tool functions as a conventional variable speeddrill. On the other hand, the value of capacitor C2 is preferablyselected so that the parallel combination of capacitors C1 and C2provides a long charge and discharge cycle resulting in a duty cyclefrequency in the range of 10-50 Hz. Accordingly, when the selectorswitch 136 is closed, or in the “screwdriver” mode, the off-time periodsat low to moderate trigger settings are sufficiently long to cause apulsing of the motor 16 under load.

Consequently, the control circuit in FIG. 9 provides the operator withthe option of operating the tool in either a conventional drill mode orin a continuous low frequency mode more suitable for controllablydriving and setting screws.

Moreover, it will be appreciated that the operation of the presentcontrol circuit in the low frequency screwdriver mode is readilycompatible with the typical operating technique employed by users ofconventional drills when driving screws. Specifically, when using aconventional variable speed drill to drive a screw, the operator willtypically drive the screw initially at or near full power and thengradually release the trigger to slow the motor to carefully seat thehead of the screw to its proper depth. This same technique is readilyusable with the control circuit of the present invention whencontinuously operated in the low frequency screwdriver mode. Inparticular, when operated with the trigger at or near its fullyretracted position, the high duty cycle setting supplies essentiallycontinuous power to the motor. As a result, the motor rotates smoothlyto rapidly drive the screw. Thereafter, as the trigger is released andthe duty cycle reduced, the motor begins to ratchet under load, therebycausing incremental rotation of the screw. As previously described, thepulsing action of the motor provides the operator with the ability tomore accurately control the final depth to which the screw head is set.Consequently, the operator is not required to learn a new technique inorder to effectively use the tool when equipped with the presentcontroller.

Obviously, if it is desired to reduce the cost of the control circuiteven further, it is possible to eliminate the selector switch 136 andcapacitor C2 and use a single large value capacitor for C1 so that thecontrol circuit always operates in a low frequency mode. Such a controlcircuit may, for example, be suitable for an inexpensive rechargeablescrewdriver.

Referring now to FIG. 12, a further simplified version of the controlcircuit according to the present invention is shown. This embodiment ofthe present controller includes a second variable resistor R2, inaddition to the trigger resistor R1, for selectively varying thefrequency of the duty cycle control signal within a predetermined range(e.g., 10 Hz-3 KHz). The trigger resistor R1 controls the percentageduty cycle of the output signal from pin 3 that controls the conductionof the FET 132 in the same manner as that described in connection withthe embodiment shown in FIG. 9. The values of resistor R3 and variableresistor R2 are selected so that the voltage signal (V_(in)) at thewiper terminal of variable resistor R2 can be varied betweenapproximately 0 volts and ⅓(+V) minus 2 diode drops. To understand themanner in which variable resistor R2 controls the frequency of the dutycycle signal at output pin 3, it is helpful to view the base-emitterjunction of the transistor 138 as a diode (designated by a phantom lineas D4) with its cathode connected to the (V_(IN)) signal at the wiper ofvariable resistor R2.

With additional reference to FIGS. 13a and 13 b, when V_(in) isapproximately equal to 0 volts, the signal at V₂ switches between adiode drop above and a diode drop below 0 volts as the duty cyclecontrol signal on output line 140 cycles on and off. Consequently, thereis little effect on the charge and discharge periods of capacitor C1 andthe frequency of the duty cycle signal is approximately equal to0.7/(R1×C1). (FIG. 13a). However, when the V_(in) signal is increased toapproximately its maximum value of ⅓(+V) minus 2 diode drops, the signalat V₂ now switches between V_(in)(+)1 diode drop to (−)1 diode drop.Thus, since the switching thresholds of the 555 timer 130 are ⅓(+V) and⅔(+V), the capacitor C1 need only charge and discharge by a very smallamount to cycle between the ⅓(+V) and ⅔(+V) thresholds. As a result, thecharge on the capacitor C1 cycles between the two switching thresholdsat a significantly faster rate, thereby increasing the frequency of theduty cycle control signal on line 140 accordingly. In short, thefrequency control resistor R2 permits the operator to vary the frequencyof the PWM control signal between a predefined low frequency (e.g., 10Hz), determined by the values of R1 and C1, and a very high frequency(e.g., 500 KHz) essentially limited by the maximum switching frequencyof the 555 timer 130. In the preferred embodiment, however, the value ofresistor R3 is selected so as to limit the maximum frequency of the PWMcontrol signal to approximately 3 KHz.

Thus, it will be appreciated that the embodiment illustrated in FIG. 12provides the operator with enhanced flexibility at a modest incrementalcost. For example, to operate the tool in a conventional drill mode, thefrequency knob R2 is adjusted to its highest frequency setting.Alternatively, to provide the most effective ratcheting mode, such asmay be needed to set very large screws, the frequency knob R2 isadjusted to its lowest frequency setting. In addition, the operator hasthe option of selectively setting the frequency knob R2 to any positionin between suitable, for example, to setting smaller screws, or forproviding a combination of conventional and pulse control with changesin the duty cycle of the control signal as described above.

With all of the embodiments described herein, it will be appreciatedthat the ratcheting or low frequency mode of operation could easily beemployed, with little or no modification, to break loose tightly seatedwood screws, nuts, etc. where the continuous application of torqueproves ineffective. It will also be appreciated that the ratcheting orlow frequency mode of operation disclosed herein could also be adaptedwith little or no modification for a variety of power tools including,but not limited to, power rivet tools, for example, to improve theeffectiveness of such tools.

Finally, it has been demonstrated that the present invention can beimplemented in various simplified forms that are producible at very lowcost and yet significantly improve the performance and effectiveness ofthe power tool to which it is applied.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification, and following claims.

What is claimed is:
 1. A power tool having an electric motor, havingassociated therewith a response time, for driving an output spindlehaving a tool holder operatively coupled thereto, an operator actuableswitch for controlling the amount of power applied to the motor, and acontrol circuit for modulating the power supplied to the motor inaccordance with the position of said switch by varying the duty cycle ofa constant frequency, pulse width modulated (PWM) direct current (d.c.)control signal generated by the control circuit to thereby control thespeed of the motor; the improvement wherein the period of the PWM d.c.control signal generated by said control circuit is greater than theresponse time of the motor.
 2. A power tool having an electric motor fordriving an output spindle having a tool holder operatively coupledthereto, an operator actuable switch for controlling the amount of powerapplied to the motor, and a control circuit for modulating the powersupplied to the motor in accordance with the position of said switch byvarying the duty cycle of a constant frequency, pulse width modulated(PWM) direct current (d.c.) control signal generated by the controlcircuit to thereby control the speed of the motor; the improvementwherein said control circuit generates said PWM d.c. control signal at afrequency that is sufficiently low to cause said motor to provide, overa substantial portion of the duty cycle range of said control signal,bursts of torque to said output spindle that cause substantial variationin the speed of rotation of said output spindle between successivebursts of torque.
 3. The power tool of claim 2 wherein said frequency iswithin a range between 10 Hz and 50 Hz.
 4. The power tool of claim 2further including a second operator actuable device for selectivelysetting the frequency of said PWM control signal.
 5. The power tool ofclaim 4 wherein said second operator actuable device comprises a secondswitch for selectively setting the frequency of said PWM control signalto either a first high frequency greater than 1 KHz or a second lowfrequency less than 50 Hz.
 6. The power tool of claim 4 wherein saidsecond operator actuable device is adapted to selectively vary thefrequency of said PWM control signal within a range that includes 10Hz-50 Hz.